In the processing of a picture in a computer, analog data representing natural pictures are not directly processed, but converted to digital data which are obtained by digitalizing the analog data at sampling points of time. In color images, the digital data are of color data, or the combination of pattern data and color data. Such data processed in a computer are composed of a predetermined number of dots, and an image reproduced by such dots has a high reproducibility, as the number of dots per a unit area is increased. Consequently, a large capacity of a memory apparatus is required in the image processing of a computer to improve the reproducibility, and, thus, a processing time becomes longer.
In the case where image data are generated in accordance with scanning on a natural picture, or taking a picture thereof by using a video camera, etc. the number of dots and colors represented by a computer is practically limited to some extent. This limitation is the resulted of the resolution power of the input apparatus such as a scanner or a video camera on one hand, and a performance such as a memory capacity, a processing speed, etc. of a computer on the other hand.
For the purpose of reproducing a natural picture with high-fidelity, a limitless number of dots are required. However, a resolution power of human eyes is limited on the distinction among dots and colors, so that images which are natural for viewers can be presented on a display.
When images are watched by viewers, an important role is played by not only brightness and darkness, but also colors. For instance, in case where two color papers of red and blue having the same brightness are positioned to make contact with each other along respective one sides, the two color papers which are distinctively displayed by colors are represented to be one paper by monochrome display. As clear from this explanation, whether a natural picture looks natural or not is largely affected by colors.
If the number of colors which can be represented by a computer is less than that of colors which can be discriminated by human eyes, fine difference of brightness and darkness and fine mixed tone of colors can not be represented by the computer. In such a case, color nonuniformity occurs, and a natural picture can not be reproduced with high-fidelity.
on the other hand, if the number of colors which can be represented by a computer is more than that of colors which can be discriminated by human eyes, a natural picture can be represented with high-fidelity. However, even if colors look the same for human eyes, there are a lot of cases in which color data obtained from the same-looking colors are different. This is because the fluctuation of color data occurs. For the reason, the processing of data is difficult, so that raw image data are required to be stored without any processing. As a result, the amount of data becomes large. This applies a pressure on a memory in regard to a capacity thereof, and influence badly on a processing time of data.
Further, if the number of colors which can be represented by a computer is approximately equal to that of colors which can be discriminated by human eyes, no problem occurs basically. Practically, however, color characteristics at the time of representing colors on a display and color characteristics which can be discriminated by human eyes are not coincided. As a result, the same problem as in the above case occurs.